Much work has been done over the years to convert heavy hydrocarbonaceous materials to more valuable lighter boiling products. Various thermal processes which have resulted from such work include visbreaking, catalytic hydroconversion, in both a slurry and ebullating bed, fluid coking, and delayed coking.
Of particular interest in the practice of the present invention is fluid coking. In fluid coking, a heavy hydrocarbonaceous chargestock, such as a vacuum residuum, is fed to a coking zone comprised of a fluidized bed of hot solid particles, usually coke particles, sometimes also referred to as seed coke. The heavy hydrocarbonaceous material is reacted in the coking zone resulting in conversion products which include a vapor fraction and coke, which is deposited on the surface of the seed particles. A portion of the coked-seed particles is sent to a heating zone which is maintained at a temperature higher than that of the coking zone. Some of the coke is burned off in the heating zone. Hot seed particles from the heating zone are returned to the coking zone as regenerated seed material which serves as the primary heat source for the coking zone. In some fluid coking processes, a portion of hot coke from the heating zone is circulated back and forth to a gasification zone which is maintained at a temperature greater than that of the heating zone. In the gasifier, substantially all of the remaining coke on the seed material is burned, or gasified, off. Examples of U.S. patents which teach fluid coking, with or without an integrated gasification zone, are U.S. Pat. Nos. 3,726,791; 4,203,759; 4,213,848; and 4,269,696; all of which are incorporated herein by reference.
Myriad process modifications have been made over the years in fluid coking in an attempt to achieve higher liquid yields. For example, U.S. Pat. No. 4,378,288 discloses a method for increasing coker distillate yield in a thermal coking process by adding small amounts of a free radical inhibitor.
Also, U.S. Pat. No. 4,642,175 discloses a method for reducing the coking tendency of heavy hydrocarbon feedstocks in a non-hydrogenative catalytic cracking process by treating the feedstock with a free radical-removing catalyst so as to reduce the free radical concentration of the feedstock.
Notwithstanding any advantages the foregoing processes may have, there is a need in the art to be able to operate fluid coking at ever lower temperatures in order to increase the conversion of the feed to more desirable products at the expense of less desirable products such as gas and coke. Unfortunately, as is known in the art, if the temperature of a thermal conversion process is decreased so as to increase the conversion of feed to more desirable products, it is usually necessary to increase the residence time of the feed in the reactor. Increased residence time, in a reactor of constant volume results in lowering of the production rate, which is undesirable. Decreasing the temperature of a thermal conversion process can also have other undesirable effects. For example, in fluid coking, lower temperature conversion can lead gross agglomeration of the fluid bed of coke particles, and the bed of coke particles becomes unstable because of the lower cracking rate of the resid feed. On the other hand, if the temperature of the conversion process is raised, the production rate will increase but at the expense of forming less valuable gaseous products, such as coke and products boiling below 100.degree. F. Thus, there remains a need in the art for increasing the rate of thermal conversion processes without forming less desirable products and preferably increasing both the rate of conversion and the yield of desired products.